Bi-stable thermal control valves that open or close when fluid conducted therethrough reaches a predetermined temperature are known and are commonly employed as over temperature shut-off valves. For example, pneumatic thermal bi-stable valves are employed in aircraft bleed air systems for cabin pressurization, air conditioning, system cooling, anti-icing, and other such purposes to ensure that hot bleed air is diverted from, for example, temperature sensitive aircraft components (e.g. the wings).
Known pneumatic thermal bi-stable valves employ thermal fuse plugs made of eutectic alloys that abruptly melt at a predetermined temperature. Generally, eutectic fuse plugs are positioned within pneumatic thermal control valves to prevent the movement of a spring-loaded flapper, a body capable of swinging between an open position that permits airflow and a closed position that blocks airflow. When bleed air heats the eutectic fuse plug to its melting point, the plug melts away and the flapper is permitted to swing shut (i.e. trip) and obstruct airflow.
Though thermal control valves of the type described above are effective for decreasing airflow when bleed air reaches a particular temperature, the eutectic plugs they employ must be replaced after melting before a tripped flapper can be reset to its open position. This requires valve removal, valve disassembly, insertion of a new eutectic plug, and valve reassembly, a time-consuming and expensive process. Furthermore, such control valves cannot be tested to ensure proper operation without heating and melting the eutectic plug.
It should thus be appreciated that it would be desirable to provide an improved thermal control valve capable of being reset and reused without valve removal and part replacement.